This project focuses on the regulation of medullary interstitial potassium and salt concentrations under normal conditions and in the presence of altered body potassium balance and ADH (vasopressin) levels. Studies will utilize the method of perfusing isolated tubule segments of both specific pathogen-free Sprague-Dawley rats and Brattleboro rats with diabetes insipidus. Although these animal models have been extensively studied previously utilizing other research techniques, the application of the isolated tubule technique in the rat has been limited. This new development will allow direct correlation of results of in vitro perfusion with previous results from in vivo micropuncture of the same species. Methodologies will involve measurement of net and unidirectional ion fluxes and transepithelial voltages in isolated, perfused segments of the loop of Henle. Dietary and drug protocols will be used to develop chronic potassium depletion and chronic potassium loading, as well as physiologic replacement of ADH in the Brattleboro rat. Initial studies will examine potential mechanisms whereby potassium may be secreted into the descending limb of the loop of Henle as has been observed by in vivo micropuncture, and whether any of these mechanisms are affected by potassium loading which has been shown to greatly augment the delivery of potassium to the tip of the loop of Henle. Recent evidence also indicates that potassium movement across the apical membrane of the thick ascending limb is coupled to salt absorption by a common transport mechanism. The effect of ADH on net salt absorption in the medullary thick ascending limb will be exmined in control and in potassium-depleted Sprague-Dawley rats in order to define ways in which hypokalemia leads to concentrating defects. Finally the Brattleboro rat will be used as a model of diabetes insipidus with dimiminished responsiveness to exogenous ADH. Studies will examine the effect of ADH on potassium transport; correlations between chronic administration of AHD and changes in the morphology and rate of salt absorption in the medullary thick ascending limb will be examined. The results of these studies should expand our understanding of the interrelationship of potassium and ADH, and their role in regulating solute transport in health and disease.